70 University of California Publications in Zoology [VOL. 13 



end. The diameter remains fairly constant until just below the mid- 

 portion, whence it gradually diminishes, to end in a blunt point just 

 below and to the right of the posterior contractile vacuole (post. c. v., 

 pi. 4, fig. 3). At the mid-part on the dorsal side of the macronucleus 

 is a shallow depression in which the micronucleus is held. The macro- 

 nucleus is entirely surrounded by a definite nuclear membrane. This 

 membrane conforms to the shape of the macronucleus and forms for 

 it a well-defined capsule. Between the membrane and the macro- 

 nucleus proper is a clear space in which no structures are visible even 

 under the most powerful magnifications (2800 to 3400 diameters). 

 The nuclear membrane is enclosed on the outside by ectoplasm, the 

 reticular structure of which is somewhat more regularly arranged 

 over the membrane (see pi. 4, figs. 3, 5). The position of the macro- 

 nucleus within the body is absolutely fixed. No evidence of mobility 

 such as has been suggested for the macronucleus of Dasytricha by 

 Schuberg (1888), or of changes in its position as suggested for some 

 species of Opkryoscolex by Eberlein (1895), and Giinther (1899), has 

 been found in D. ecandatum. This absolute fixation of the macro- 

 nucleus in D. ecaudatum is undoubtedly brought about through its 

 close relation to the skeletal structure, to the right edge of which 

 it appears to be firmly connected (pi. 3, fig. 1; pi. 4, fig. 5; pi. 7, figs. 

 26-30). The skeletal structure, therefore, in this place functions as 

 a supporting structure for the macronucleus. The microphotographs 

 (figs. 27, 28) show this especially well. It is desired to call attention 

 in this place to the fact that a careful study of the best preparations 

 gives no evidence which tends to show that there is any direct com- 

 munication between the oesophageal wall and the nuclear membrane 

 or that the macronucleus has any special supporting structures other 

 than the right edge of the skeletal structure and the surrounding 

 boundary layer and ectoplasm. The microphotographs (pi. 6, fig. 13; 

 pi. 7, figs. 26-30, 33) might indicate otherwise, but this is because the 

 sections were cut somewhat obliquely and hence there is a slight super- 

 imposition of some of the structures. Hence it can be emphatically 

 stated that structures homologous with the "Kernstiele" of Schuberg 

 (1888), Eberlein (1895), and Giinther (1899) are not present in D. 

 ecaudatum. The macronucleus itself is distinctly granular. After 

 iron-alum haematoxylin stain these granules stand out clearly and 

 distinctly and may, in thin cross-sections, be counted (mac., pi. 4, figs. 

 3 and 5). The estimated total number (based on examination of three 

 specimens) was approximately 25,000 granules. Interesting changes 



